Guidance around branching strategy for maintenance releases on older SDK versions

[QuintinWillison]

As part of our commitment to adopting semantic versioning across all SDKs from version 2, we are going to be making more breaking changes to SDKs. This brings with it the expectation that we will need to release patch releases on older series of SDKs, most likely in response to bugs reported by customers.

We should document how this will achieved, including naming convention for these maintenance branches. The expectation is that we would not created a maintenance branch for an older SDK version until the need arises (branch from Git release tag), and that at that point that branch would become protected in the same way that main branch is.